This invention generally relates to fuel compositions. Some specific embodiments of the invention are directed to methods for preparing liquid fuels from biomass or municipal solid waste materials.
In view of the projected, long-term shortages in the availability of quality fossil fuels, there has been tremendous interest in the development of renewable sources of fuels. One of the most attractive sources for such fuel is biomass, which can be used to prepare a variety of different types of fuel—some of which are referred to as “biofuel”, or “biodiesel”. Another potential source is municipal solid waste (MSW), which usually contains primarily household waste, but which can also include commercial waste.
There are two main routes for producing liquid fuels from biomass materials. The indirect route involves biomass gasification. In such a process, the raw material is gasified under partial combustion conditions, to produce a syngas based on carbon monoxide and hydrogen. Air-blown circulating fluidized bed (CFB) gasifiers are often well-suited for small-scale biomass gasification. The syngas can then be converted into a liquid fuel by way of Fischer-Tropsch (FT) synthesis.
While the indirect method is useful in many situations, it often requires very high temperatures, e.g., 800° C.-1,700° C., depending on the type of gasifier. There may also be difficulties in reliably feeding the raw material into the pressurized gasifier. Moreover, for the CFB processes, nitrogen dilution can be problematic. Also, high tar concentrations in the product gas often necessitates subsequent gas clean-up steps, which can increase capital costs.
Pyrolysis is another method for producing the liquid fuels from biomass, and this technique can be thought of as a “direct method”. The process itself is known in the art, and involves the thermal decomposition of biomass or other carbonaceous materials. The process is carried out in the absence of oxygen, or in the presence of significantly reduced levels of oxygen, as compared to conventional combustion processes. The temperatures involved are much lower than for gasification, e.g., about 400° C.-600° C. The primary products of pyrolysis are oils, light gases, and char. As further described below, the vapor products of pyrolysis can be condensed to a liquid product, i.e., a “bio-oil”, by condensation, for example.
Bio-oils (“pyrolysis oils”) are valuable fuel precursors, but they are also quite distinct from hydrocarbon-based petroleum fuels. The high oxygen content of the pyrolysis oils, e.g., up to about 50% by weight, would take such materials outside the conventional definition for a hydrocarbon. These relatively high levels of oxygen limit the use of the compositions, in applications such as transportation fuels (gasoline and diesel fuel). In most instances, the oxygen content would have to be reduced considerably, to allow additional upgrading steps to form the conventional fuels.
In view of these considerations, new processes for preparing fuels, e.g., liquid fuels, from biomass or MSW materials would be welcome in the art. For many end use applications, the processes should be based in part on pyrolysis reactions. Moreover, they should minimize the amount of oxygen content found in one or more of the pyrolysis products. The new processes should also be capable of economic implementation, and should be compatible with other procedures, e.g., fuel upgrading steps of the pyrolysis products.